Novel Deep Eutectic Solvent-Dissolved Molybdenum Oxide Catalyst

Article pubs.acs.org/IECR

Novel Deep Eutectic Solvent-Dissolved Molybdenum Oxide Catalyst for the Upgrading of Heavy Crude Oil S. M. Shuwa, R. S. Al-Hajri,* B. Y. Jibril, and Y. M. Al-Waheibi Petroleum and Chemical Engineering Department, Sultan Qaboos University, P.O. Box 33, PC 123 Muscat, Oman S Supporting Information *

ABSTRACT: A MoO3/deep eutectic solvent (DES) catalyst precursor solution was prepared by dissolving molybdenum trioxide in a DES based on choline chloride/urea. The catalyst precursor solution was characterized for its physical and chemical properties. The characterization results showed almost no change in the DES properties after the addition of MoO3. The solution was used in the catalytic upgrading reaction of heavy crude oil. The performance of the catalyst was analyzed by gas chromatography−mass spectrometry, Fourier transform infrared, and viscosity measurements of the heavy oil before and after the reaction. The use of the catalyst in the catalytic aquathermolysis showed an increase in the oil viscosity. In the presence of hydrogen and catalyst, the results showed a 43% reduction in the heavy oil viscosity, a 2.5° increase in the API gravity, and 32 wt % sulfur reduction.

1. INTRODUCTION Heavy oil and bitumen require energy-intensive operations for their production, upgrading, and transportation to refineries for subsequent production operations. Therefore, reducing the viscosity of the heavy oil simultaneously with thermal enhanced oil recovery has attracted a lot of attention in the past few years. Moore et al. and Weissman et al. were among the first to propose the concept of in situ catalytic upgrading of heavy oil during in situ combustion.1−3 Tian et al.4 used water-soluble ammonium heptamolybdate and NiNO3 in upgrading residual oil from the Petronas Refinery in Malaysia. The upgrading reactions were conducted in a batch mode under a pressure of 7 MPa of H2 and 340 °C using a 300 mL high-pressure stirred reactor. The catalyst increased the hydrogen-to-carbon ratio from 1.46 to 1.86 (27.9% increment) with 62.2% viscosity reduction. Molybdenum acetylacetonate and iron alkylhexanoate based oil-soluble catalysts offered about a three-fold reduction in the viscosity, a 9° increase in the API gravity, and 46 wt % sulfur reduction.5 The experiments were conducted in batch (Parr Instrument Company) stirred reactor of 1800 mL capacity at a temperature of 400 °C in a hydrogen atmosphere of a final pressure of 10.8 MPa, at a stirring speed of 1000 rpm, catalyst loadings of 10 wt % with respect to oil, and a residence time of 24 h. The process in which the viscosity of heavy oil is reduced with the aid of water is called aquathermolysis. Not all of the catalysts reported having good catalytic activity on the heavy oils. In some cases, the viscosities of reacted heavy oils regressed rapidly after reaction.6,7 This is the reason why researchers applied hydrogen or hydrogen-donor compounds in heavy oil upgrading. The process that involves hydrogen in aquathermolysis is called hydrothermolysis, which is a terminology used to distinguish it from aquathermolysis.8 Chao et al. used a bifunctional catalyst, alkyl ester sulfonate copper, which has not only a catalytic center but also a hydrogen precursor structure. The catalyst showed good activity in aquathermolysis of heavy oil in both field and © XXXX American Chemical Society

laboratory tests. The laboratory results showed 90.72% reduction in the viscosity using 0.3 wt % catalyst at 240 °C and 24 h, with 10.12% conversion of heavy content to light content.6 Mohammed and Mamora9 carried out an experimental study on the in situ upgrading of a local Venezuelan (Jobo) heavy oil under steam injection at a temperature of 273 °C and a pressure of 500 psig using an oil-soluble organometallic catalyst at a concentration of 750 ppm and tetralin as the hydrogen donor. The results showed an increase in oil recovery by 15% by adding 5 wt % tetralin above that of pure steam injection. When the oil was mixed with tetralin, a catalyst solution, and sand, about 20% higher oil recovery than that of pure steam injection was observed. In another work,7 the effect of a hydrogen-donor additive on the viscosity of heavy oil during steam stimulation was investigated. The results showed that the incorporation of tetralin (0.8%) as a hydrogen donor led to a viscosity reduction of 80% after 24 h of reaction time at a temperature of 240 °C. The shortcomings of water-soluble catalysts are a low surface area-to-volume ratio, which leads to inefficient contact between the catalyst and feed, and the production of particles of bigger sizes, which can lead to formation damage in the reservoir. Oilsoluble catalysts are expensive, which prompted researchers to seek other alternatives. Recently, some researchers have employed the services of ionic liquids to upgrade and enhance the recovery of heavy crude oils. Nares et al. conducted a batch reactor study of upgrading a heavy crude oil from the Gulf of Mexico using ionic liquids elaborated with iron and molybdenum. The upgrading experiments were conducted using ionic liquids based on iron (10 wt %) and molybdenum (2 wt %) compounds, in a liquid phase homogeneously mixed with heavy crude oil in a batch reactor of 500 mL, at 400 °C Received: December 28, 2014 Revised: March 14, 2015 Accepted: March 23, 2015

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DOI: 10.1021/ie5050082 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

24 h. The pure DES, DES + H2O mixture (the mass ratio of DES to water is 2:1), DES/MoO3 solution, and DES/MoO3 + H2O (mass ratio of DES/MoO3 to water is 2:1) aqueous mixtures were formed and characterized at different temperatures for density, viscosity, conductivity, and surface tension. The density and API gravity measurements were performed using an Anton Paar vibrating tube densimeter (DMA 4500). The equipment measures periods of harmonic oscillation of a built-in U-tube made of borosilicate glass containing the sample. The temperature of the measuring cell is controlled by an integrated Peltier thermostat. Before each measurement, the cell was rinsed with deionized water and slowly evacuated to avoid any trapped air in the system. Then the inlet valve was opened and the sample introduced into the cell; at least 10 min was allowed for the temperature to reach stability, and then measurements were taken. The API gravity at 15 °C was measured using the API method developed in the instrument. The instrument computed the API gravity automatically from the specific gravity values at 15 °C. The viscosity measurements were carried out with an Anton Paar rheometer (Rheolab QC) at a constant shear rate of 300 s−1. The instrument had a builtin temperature sensor, and an external water bath was used for temperature control. For every measurement, about 14 mL of the sample was put into the sample cell with its spindles and connected to the instrument. About 15 min was allowed for temperature equilibration before measurements were taken. The electrical conductivity was measured using a Metler Toledo conductivity meter, which operates with an alternating current of 60 Hz frequency. The conductivity meter was calibrated using a KCl reference solution. The instruments had a built-in temperature sensor, and variation of the temperature was achieved with a water bath. For each measurement, 5 mL of sample was used, the conductivity sensor was immersed in the glass vials containing the samples, and the conductivity values were displayed on the instrument’s digital screen. After every measurement, the conductivity cell was washed with deionized water and acetone to remove any adhering sample and dried before using it in the next measurement. The surface tension measurements were carried out using a Kruss digital tensiometer (K10ST) by the Du Nouy ring method.19 An external water bath was connected to the tensiometer for temperature control. The platinum−iridium ring was cleaned by flaming, and the glassware was rinsed consecutively with acetone and distilled water before each measurement. The equipment calibration was determined by measuring the surface tension of pure water. After each measurement, the glassware was cleaned thoroughly with water and acetone before the next measurement. Prior to these measurements, a solubility test of the metal oxide in the DES was conducted. Amounts of 0.08−0.2 g of MoO3 at 0.02 g intervals were dissolved in 10 mL of DES and shaken in the thermoshaker for 24 h at 80 °C. The mixtures were then observed for the formation of clear homogeneous solution of the oxide and DES. A clear homogeneous solution is an indication of complete dissolution of the metal oxide in the DES. Thermal and structural studies were undertaken using Fourier transform infrared (FTIR) and thermogravimetric analysis (TGA) to see if there was any change caused to the DES after the incorporation of MoO3 into the DES structure. TGA of the DES and DES/MoO3 was conducted on a simultaneous thermal analyzer (PerkinElmer, STA 6000). The samples (10−20 mg) were heated from 30 to 500 °C at a

under a 10.8 MPa total pressure of hydrogen with residence times of 24, 48, and 72 h. The oil was successfully upgraded because there was substantial reduction in the viscosity and sulfur content and increase in the API gravity.10 Fan et al. investigated the potential of metal-modified ionic liquids in upgrading Liaohe heavy crude oil, resulting in a good viscosity reduction property and possibly leading to high oil recovery.11 Deep eutectic solvents (DESs) are types of solvents that belong to the family of ionic liquids but with a special property composition of two or three cheap and safe components that are capable of self-association, often through hydrogen-bonding interactions, to form a eutectic mixture with a melting point lower than that of each individual component.12 DESs are generally liquid at temperatures lower than 100 °C and belong to a category of green solvents that have vast applications in catalysis, organic synthesis, electrochemistry, and material chemistry.12,13 They are another form of ionic liquids that are cheaper, greener, and easier to prepare compared with typical ionic liquids. Molybdenum is the basic constituent of the most active hydroprocessing catalysts.14−17 The major source of molybdenum is the mineral molybdenite (crystalline molybdenum sulfide, MoS2), which is roasted to produce molybdenum oxide and purified by dissolution in aqueous ammonia to produce molybdates such as ammonium heptamolybdate and ammonium dimolybdate, which are further purified by fraction crystallization and flash evaporation at 100 °C, respectively, to obtain the water-soluble catalyst precursors. Thus, ammonium heptamolybdate and ammonium dimolybdate are among the major sources of water-soluble molybdenum catalysts. Metal oxides can be introduced into the structure of some DESs like choline chloride/urea through dissolution to form catalyst precursor compounds. An active dispersed catalyst precursor produced from this material in a simple process could have low cost compared to oil-soluble catalyst precursors. It could also solve problems associated with water-soluble precursor compounds such as the generation of large catalyst particle sizes, and the use of excess amounts of water in the feedstock will be avoided, which is undesired in hydrocracking. The main purpose of this work was to investigate the performance of new catalysts based on DESs in the upgrading of a heavy crude oil. The catalyst is expected to lead to a reduction in the viscosity and sulfur content and an increase in the API gravity of the original oil. The operation conditions for the reaction were chosen to model typical reservoir conditions. To our knowledge, the use of DES/molybdenum trioxide in the aquathermolysis/hydrocracking reactions has not been reported. It is the objective of this work to investigate its potential in upgrading Omani heavy oils.

2. EXPERIMENTAL PROCEDURES 2.1. Catalyst Synthesis and Characterization. A sample of DES was synthesized using choline chloride (ChCl) and urea (molar ratio of 1:2). The synthesis was conducted in an incubator shaker at 80 °C. Appropriate quantities of the salts were weighed, thoroughly mixed, put into the shaker, and allowed to completely melt to a homogeneous colorless liquid. Details of the synthesis have been reported elsewhere.18 In order to form a solution of the DES and molybdenum trioxide (MoO3), a known quantity of the oxide (to give 1000 ppm of molybdenum with respect to the crude oil) was measured, put in a sample vile containing the DES (10 wt % with respect to the oil), and placed in the thermoshaker operated at 80 °C for B


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film of prolene. The FTIR spectrum was obtained by placing a drop of the samples between two sodium chloride cells and then spreading on the surface of the cells. The cells were then mounted on the equipment and the settings were as follows; 14 scans, at a resolution of 4 cm−1, and wavenumbers from 400 to 4000 cm−1. The GC−MS spectrum was obtained using a Clarus 600 chromatograph (PerkinElmer) with a RTX-5MS column (30 m × 250 μm × 0.25 μm). For nonvolatile polar components of oil (asphaltenes and resins) removal, solvent precipitation and column separation were first conducted. This is to prevent the components from damaging the column. The solid precipitate recovered from run S3 was further characterized using X-ray diffraction (XRD) and scanning electron microscopy−energy-dispersive spectroscopy (SEM− EDS) techniques. The diffractogram was obtained with a PanAnalytical (XpertPro) XRD machine. The SEM image and elemental analysis of the samples were obtained with a JEOL electron microscope with built-in EDS technology for elemental determination.

heating rate of 10 °C/min under a nitrogen atmosphere (30 mL/min flow rate) at a pressure of 3 bar. 2.2. Heavy Oil Upgrading Experiments. To investigate the activity of the proposed MoO3/DES system, a heavy oil sample from an Omani field that has a viscosity of 13800 cP was utilized. Upgrading experiments of the heavy crude oil were carried out using a 500 mL capacity batch-type laboratory reactor (4575 series, Parr Instrument Company). The reactor vessel was equipped with a magnetic stirrer for mixing samples in the vessel, a dip tube for liquid and gas sample withdrawal from the vessel, and a thermowell. The thermocouple was positioned inside the reactor through the thermowell to monitor the temperature of the samples inside the vessel. The temperature and stirring speed are controlled automatically by a digital controller (4848 series, Parr Instrument Company), while the pressure was monitored with a pressure gauge mounted on the reactor head connected to the reactor vessel. The following constant reaction conditions were utilized: 300 °C reaction temperature, 11 bar initial hydrogen pressure, 24 h reaction time, and 750 rpm stirring speed. The hydrocracking experiments were conducted under a hydrogen atmosphere by heating 100 g of a heavy oil sample in the reactor vessel with and without catalysts, while the aquathermolysis runs were conducted under a nitrogen atmosphere with a heavy oil sample containing 30 wt % water (oil-to-water mass ratio of 70:30), i.e., 100 g of oil to 43 g of water. The catalyst loading used was 1000 ppm (0.1 wt % molybdenum) and 10 wt % DES with respect to the heavy oil. Table 1 gives the breakdown of all of the runs and

3. RESULTS AND DISCUSSION 3.1. Characterization of Catalyst Precursor Solutions. Physical properties like density, viscosity, refractive index, and surface tension are important characteristics of DESs. To date, few works are available in the open literature on this area of interest.13,18,20−25 It is imperative to know the characteristics of the DES and its aqueous mixtures in order for them to be applied in industrial and chemical processes. Because the DES was mixed with molybdenum oxide and the fact that water is one of the major components found in oil reservoirs, the DES (pure) and DESs dissolved in MoO3 and diluted with water were formulated and characterized. These characteristics could provide information on the purity of the precursor solutions and molecular interaction in the liquid.23 Table 2 gives the

Table 1. Experimental Runs and Conditions Employed sample

H2

N2

H2O

catalyst (DES/ Mo)

S1 S2 S3 S4 S5 S6 S7

comments control sample: fresh heavy oil

√ √ √ √

Table 2. Retention Time of Standard Paraffinic Hydrocarbons (C8−C33)

√

√ √

√ √ √ √

C8 C9 C10 C11 C12 C13 C14 C15 C16

√ √

conditions employed for the reactions. Naphthalene was used as the model oil along with water under a nitrogen atmosphere to check for any thermal activity. Less than 4% naphthalene conversion was noticed after 24 h and 300 °C operating temperature. The oil plus the catalytic precursor was then mounted on the reactor assembly and the reactor purged several times with nitrogen to remove air from the vessel. Heating was then started from room temperature to the desired reaction temperature. The reaction began when the desired temperature was reached and stirring was initiated. The temperature and pressure were monitored and recorded continuously during the reaction. After the reaction was completed, the reaction mixture was cooled to around 70 °C. For tests containing water, the water was decanted first and the liquid products from the runs were recovered and analyzed for the density and API gravity using an Anton Paar vibrating tube density meter and the viscosity using an Anton Paar rheometer. The elemental sulfur content of the runs was evaluated with an energy-dispersive X-ray fluorimeter (NEX QC+ Rigaku) using 32 mm single sample cup with a thin

2.658 3.049 4.249 5.65 7.09 8.471 9.781 10.942 12.177

C17 C18 C19 C20 C21 C22 C23 C24 C25

13.283 14.328 15.324 16.274 17.185 18.055 18.89 19.686 20.456

C26 C27 C28 C29 C30 C31 C32 C33

21.201 21.912 22.607 23.287 24.052 24.938 25.998 27.199

results of these characteristics with experimental errors at different temperatures. The result of the solubility test showed MoO3 dissolved in the DES, with 0.16 g of MoO3 soluble in 10 mL of DES; this is equivalent to 16000 ppm. A solution of 18000 ppm (0.18 g of MoO3 in 10 mL of DES) did not form a clear homogeneous phase, which is an indication of the limited solubility of the oxide in the DES at that condition. The 16000 ppm solution formed a clear homogeneous mixture, which indicates complete dissolution of the oxide in the DES. Figure 1 is a plot of the density as a function of the temperature for all of the samples. From the graph, a decrease in the density with temperature can be seen, as expected with a linear relationship. However, the incorporation of molybdenum oxide into the DES structure did not increase its density, but rather a slight decrease was observed. The values are in C


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structure, which led to an increase in the mobility of free species within the DES. Figure 3 shows variation of the surface tension as a function of the temperature. Almost similar behavior and trend were

Figure 1. Densities of catalyst carriers (DES) and catalyst precursors with and without water as a function of the temperature.

agreement with the values reported in the literature.22,23 The reduction in the density due to the addition of water to pure DES and DES/MoO3 at all temperatures was found to be proportional. Viscosity is an important property of DES and is a strong temperature-dependent variable. Figure 2 displays the vis-

Figure 3. Surface tensions of catalyst carriers (DES) and catalyst precursors with and without water as a function of the temperature.

observed with viscosity. The surface tension also depends on the strength of intermolecular forces between the species in the structure and, as a result, strongly correlates with the viscosity. The addition of water to the DES led to a drastic reduction in the surface tension. Conductivity is also an important property of DESs. Because ChCl/urea consists of ionic species that are dissociated in the liquid and are free to move independently,26 it is expected to have some degree of conductivity. Figure 4

Figure 2. Viscosities of catalyst carriers (DES) and catalyst precursors with and without water as a function of the temperature.

cosities of the various solutions as a function of the temperature. The incorporation of MoO3 in the structure of the DES led to an increase in the viscosity. This is due to an increase in the strength of hydrogen bonding between components in the structure because the viscosity is known to be associated with hydrogen-bonding interactions. A 2:1 eutectic mixture of urea with ChCl has ionic components of choline cations and chloride anions while having hydrogen bonding of the urea molecules. MoO3 binds to the urea molecules or to the chloride anion. There is also the possibility of forming an anion/metal/urea complex. The presence of a strong coordinating anion is needed for the complexation of metal oxide to form soluble species with a ChCl/urea mixture.18 The addition of water led to a reduction in the viscosity as expected because the viscosity is also a function of the composition. This may be associated with distortion of extensive hydrogen bonding between the components in the

Figure 4. Conductivities of catalyst carriers (DES) and catalyst precursors with and without water as a function of the temperature.

displays this behavior. When water was added to the DES and DES/MoO3 solutions, the conductivity increased as observed. This can be attributed to the increase in the mobility of the ionic species in the system, especially the chloride anion (being the principle migrating species), which is more facile than the choline cation or the urea anion. It can also be observed from Figure 4 that the introduction of MoO3 into the DES led to a slight decrease in the conductivity. It has been reported that D


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Industrial & Engineering Chemistry Research DESs with high viscosity have low conductivity.12 Because the viscosity correlates inversely with the conductivity, the increase in the viscosity observed after adsorption of MoO3 into the DES structure led to a reduction in the conductivity. TGA for both pure DES and DES/MoO3 is shown in Figure 5. It is evident from the thermograms that the DES and DES/

molybdenum oxide. This peak could be assigned to the hydrogen bond N−H of urea in the DES, with the chlorometalate complex species anion expected to have been formed because of the presence of the metal oxide.29,30 It is assumed that there were hydrogen-bonding interactions between MoO3 and the DES, as shown in the FTIR spectra. The transmission bands at 3771 and 3825 cm−1 are attributed to hydrogen bonding between urea and the complex anion expected to have been formed due to dissolution of the metal oxide in the DES. Although there is no work in the open literature carried out on dissolving MoO3 in ChCl/urea DES, Abbot et al.18 dissolved V2O5 and CrO3 in ChCl/urea DES. These two metals/metal oxides exhibit some common similarities with MoO3. Molybdenum and chromium are in the same group in the periodic table with many similar chemical properties. They have high oxidation states (6+) in the two oxides and, as a result, possess good catalytic activity. V2O5 also exists in high oxidation state (5+) and possesses strong catalytic activities like MoO3 and CrO3. All three metal oxides are oxidizing agents. Considering the above-mentioned similarities common to the three metal oxides, MoO3 dissolved in ChCl/ urea DES is expected to exhibit behavior similar to that of CrO3 and V2O5 in ChCl/urea DES. The FTIR spectra confirmed this assertion by not showing any new bands apart from those attributed to hydrogen-bonding interactions between the urea and the complex anion species. It can be speculated that dissolution of MoO3 in the DES led to the formation of a complex anion species such as [MoO2Cl3]− similar to [CrO2Cl3]− and [VO2Cl2]− obtained when CrO3 and V2O5 were dissolved in ChCl/urea DES, respectively.18 The oxidation state of molybdenum in the complex anion is 6+, which means no reduction after dissolution. There is a possibility that the oxide could be reduced in the dissolution process; however, this was not observed because the solution of the oxide in the DES remained colorless even after dissolution, which is a characteristic of the metal in its “6+” oxidation state. A similar characteristic was observed when CrO3 and V2O5 were dissolved in ChCl/urea DES18 and V2O5 in ionic liquid31 in spite of the fact that the two metal oxides are stronger oxidizing agents than MoO3. Many metal oxides do not undergo reduction after dissolution in ChCl/urea DES.32 This points out that the major outcomes of dissolving metal oxide in

Figure 5. Thermogram of the samples.

MoO3 are thermally stable up to around 200 °C. This agrees with the observation reported in the literature.27,28 The maximum decomposition temperature was found to be 295 °C for the two samples analyzed. The presence of MoO3 in the DES structure did not change the thermal stability of the solvent or the maximum decomposition temperature, with only a small difference in the amount of residue left after decompositions of the compounds at 295 °C. The IR spectrum was acquired to see if there was any change in the structure of the DES after the introduction of molybdenum oxide. Figure 6 shows the IR spectrum of the pure DES and DES/MoO3. Upon comparion of the two spectra in the figure, it can be seen that two new transmission bands appeared at 3825 and 3771 cm−1 in the spectrum of DES/ MoO3. This was not observed in the spectrum of pure DES, indicating the interaction of pure DES with dissolved

Figure 6. FTIR spectrum of DES before and after the incorporation of MoO3. E


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Figure 7. GC−MS spectra of samples S2 and S3 (hydrocracking without water) in comparison with the fresh sample S1.

Figure 8. GC−MS spectra of samples S4 and S5 (hydrocracking with water) in comparison with the fresh sample S1.

This may be attributed to the formation of more hydrogen bonds between urea and ChCl because of the presence of MoO3.35,36 Because the viscosities of ionic liquids and ionic liquid analogues are governed essentially by hydrogen bonding,37 it is reasonable to assume that an increase in the hydrogen-bonding interaction and its strength is the reason behind the increase in the viscosity when MoO3 was dissolved in the ChCl/urea DES. 3.2. Compositional Changes. The GC−MS study was undertaken to evaluate changes in the hydrocarbon components in the oil samples before and after reactions under the studied conditions. The results are presented in Figures 7−9. The amounts of saturated hydrocarbons C8−C33 (retention

ionic liquids and DESs are the formation of a complex anion in situ, and the mechanism of interaction between the solutes and solvents is hydrogen bonding.18,29,31,33,34 On the basis of the foregoing discussions, it is expected that the oxide will be reduced in situ by reacting with the sulfur compounds in the oil to form molybdenum disulfide (MoS2), which is the active form of the catalysts. The presence of the complex anion containing molybdenum in the 6+ oxidation state and hydrogen will pave the way for reduction of molybdenum from the 6+ to 4+ oxidation state as in MoS2. Also, the absorption bands at 2161 and 2331 cm−1, which appeared in the spectrum of pure DES, shifted to broader and higher frequency bands at 2190 and 2336 cm−1, respectively. F


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Figure 9. GC−MS spectra of samples S6 and S7 (aquathermolysis) in comparison with the fresh sample S1.

This must have been as a result of polymerization reaction of the molecules in the fresh oil. The formation of high-molecularweight hydrocarbons can be explained through free-radical chain reactions caused by the absence or presence of low partial pressure of hydrogen in the reaction. Thermal cracking of asphaltenes and resins can lead to the formation of reactive species that can react with each other through polymerization and condensation reactions to form bigger molecules.38 This is the reason why there is a big difference in the saturate contents between S4 and S2 in which water was the only difference between the two starting reactive mixtures. Hydrogen present in S2 was able to stabilize the reactive free radicals generated at that condition, whereas the high partial pressures of water in S4 inhibit the free-radical stabilization. Because polymerization and condensation reactions are proceeded by free-radical mechanisms, this can lead to the production of components heavier than the feed, and as a result, low saturates will be formed compared with S2. In summary, S1 and S2 showed the significant presence of saturates because of the presence of hydrogen and catalyst (in S3), which is the result of hydrogenation and cracking reactions. S3 and S5 showed lower amounts of higher saturated hydrocarbons (C23−C33) because of their conversion to lowmolecular-weight hydrocarbons as a result of the presence of catalyst and hydrogen. S4 displayed amounts of hydrocarbons C28−C33 higher than those of the fresh oil because of condensation and polymerization reactions taking place in which the presence of water aided the reaction. For S6 and S7, even though there were low amounts of C28−C33, hydrocarbons of higher carbon number were detected from the product of the reaction. This can be attributed to the presence of water, as in S4, and the absence of hydrogen, which is known to inhibit polymerization reactions. The FTIR spectra of the oil samples before and after upgrading reactions were acquired to compliment GC−MS analysis. The results of FTIR analysis are represented in Figures 10 and 11. The transmission band at 3278 cm−1, which is assigned to alkynes and aromatic C−H stretching vibra-

time shown in Table 2) increased for all of the samples compared with the fresh oil sample (S1). A remarkable behavior is displayed by S2 and S3 by showing high amounts of saturates compared to other samples. S3 contains the highest amounts of saturates, followed by S2, which has the amounts of hydrocarbons nearly equal to those of S3 (Figure 7). See Table 3 for the total yields of the saturates. Table 3. Total Yield of Saturated Hydrocarbons (C8−C33) for All of the Samples sample

yield (wt %)

sample

yield (wt %)

S1 S2 S3 S4

2.9 8.6 9.7 3.5

S5 S6 S7

4.2 4.8 5.4

The presence of a high partial pressure of hydrogen in S2 and S3 aided the conversion of unsaturates present in the fresh sample to saturated hydrocarbons (hydrogen addition) with more of C−C cleavage (carbon rejection or cracking) taking place. The small increase in saturated hydrocarbons observed in the other samples (S4−S7) must be due to the cracking of high-molecular-weight hydrocarbons in the fresh sample to form saturates and light hydrocarbon gases. S3 showed higher amounts of saturates compared to other samples due to the conversion of bigger molecules to small hydrocarbons and even to gases that were not monitored in the course of the reaction. This is due to the presence of the catalysts in the reaction medium. Although S4−S7 displayed higher quantities of saturates than S1, hydrocarbons of higher carbon number were observed, as shown in Figure 9. In addition to the GC chromatograms, Figure S1 in the Supporting Information (SI), depicting columns showing peak areas of all of the reacted oil samples from the runs, was plotted. A comparison between the runs (S1−S7) in terms of the peak areas from the raw total ion chromatogram (TIC) can be easily made from the figure. G


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Figure 10. FTIR spectra of oil samples S2−S4 (hydrogen process) with fresh S1.

Figure 11. FTIR spectra of oil samples S5−S7 (aquathermolysis) with fresh S1.

bands of a conjugate polyene CC (1600 cm−1), which signifies the presence of an aromatic ring, became weaker in S2 and S3.6,39−41 This may be due to hydrogenation, open-cycle, and breaking reactions42 3.3. Change in the Physical Properties of Oil Samples after Upgrading Reactions. The density, viscosity, and API gravity measurements for the samples before and after reaction are shown in Table 4. The change in the viscosity and API gravity was calculated according to the following equation:

tions39,40 in unsaturated hydrocarbons, lost intensity and became weaker in S2, S3, and S5. This is an indication of the conversion of unsaturates to saturated hydrocarbons in the presence of hydrogen. The bands at 2927 cm−1 assigned to the C−H stretching vibration6,39−41 found in saturated hydrocarbons obviously gained intensity and became stronger in S2 and S3 and lost intensity to become weaker in S4 and S7. Also, the absorption bands at 1460 and 1376 cm−1 attributed to C− H bending vibrations39,41 found in alkanes became stronger in S2 and S3 and weaker in S4 and S7. This implies that the saturate content in S2 and S3 increased as a result of the hydrogenation of unsaturates and pyrolysis of long-chain alkanes, while the weakening of the bands in S4 and S7 illustrates a reduction in the content of saturates due to cracking to light hydrocarbon gases and/or condensation reactions to form bigger hydrocarbons. The stretching vibration

ΔM =

M0 − M × 100% M0

where ΔM is the change in the property [viscosity (mPa s, 30 °C) or API gravity (at 15 °C)], M0 is the initial property (viscosity or API gravity), and M is the property of the sample after reaction. H


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error. This can be explained with the fact that the aquathermolysis process involves breaking of the weak C−S bond, which results in sulfur reduction.6,8,41,45,46 Although the process can lead to desulfurization, it is not effective like the hydrocracking process. Viscosity reduction normally observed in the process was not observed in this case. It is clear from the runs that the presence of water in both hydrocracking and aquathermolysis led to the formation of components in reacted oil samples heavier than the original sample, as demonstrated in S4−S7. In addition to the products of condensation/polymerization reactions, it can be presumed that water hydrolyzes the sulfur and nitrogen compounds in the oil such as sulfoxides, aliphatic sulfides, and theophenic aromatic sulfides. The products of this hydrolysis reaction that still retain the sulfur in their molecules are normally sulfones, sulfoxides, and oximes. This may be the reason why a small reduction in sulfur was observed in runs S4−S7 compared to S2 and S3; because the elemental sulfur analyzer (XRF) used determines the sulfur content in the reacted liquid oil samples, any sulfur compound other than H2S will remain in the samples. The presence of these groups of compounds in the reacted oil samples is expected to result in a highly structured, higher viscosity oil.43 Water is also capable of hydrolyzing the aromatics in the oil to phenols. The appearance of a weakly intense band at 1600 cm−1 assigned to the stretching vibration band of conjugate polyene CC, which indicates the presence of an aromatic ring (higher intensity in S4−S6), supports that argument. This can be speculated based on the FTIR spectra of the oil samples S4−S6 compared to the fresh oil sample S1. Because these compounds formed from hydrolysis of the sulfur- and nitrogencontaining species in the oil are more polar than the starting materials (aliphatic sulfides and naphthenic aromatic sulfides such as dibenzotheophene), they cannot be detected by GC− MS analysis. The reason is because all polar compounds were removed by passing the dichloromethane-dissolved oil samples through silica gel, which, in turn, separates them from other nonpolar components of the oil samples. It can be observed in Figure S1 in the SI that the total saturate contents (C8−C33) of samples S2 and S3 is higher than that of S4−S7, as demonstrated by their peak areas. Hydrocracking is known for producing liquid distillates and as such not many heavy molecules and gaseous products. The aquathermolysis runs produced probably heavier components and other products that can be linked to the hydrolysis reactions because of the presence of water. As a result of all of these, the liquid distillate products (represented by C8−C33 in this case) of the hydrocracking runs is expected to be higher than the aquathermolysis runs, as demonstrated by the peak areas in the GC−MC results (Figure S2 in the SI). For the polymerization/condensation reactions, the presence of water under the condition studied can be presumed to favor free-radical addition reactions to free-radical cracking reactions. Because polymerization reactions proceed by free-radical mechanisms as in ethylene to polyethylene plastics, this may lead to the formation of heavier molecules in the reacted oil samples. It is the free-radical cracking reactions that normally lead to desulfurization and a reduction in the viscosity in the aquathermolysis process; although sulfur reduction cannot be rule out in this case (because a small reduction in sulfur and an increase in the saturate contents observed support that), freeradical addition reactions are more pronounced. In short, the

Table 4. Physical Properties of the Oil Samples from the Runs and the Fresh Crude Oil sample

viscosity, cP

change in the viscosity, %

change in the API gravity

density, g/cm3

S1 S2 S3 S4 S5 S6 S7

13800 10683 7889 17098 14492 17062 14973

NA −22.59 −42.83 +23.94 +5.02 +23.64 +8.5

12.75 +1.7 +2.4 −0.85 +0.40 −0.47 −1.09

0.9800 0.9769 0.9761 0.9837 0.9783 0.9837 0.9827

In general, the results agreed well with those obtained using GC−MS and FTIR. The presence of hydrogen during S2 and S3 resulted in a higher C8−C33 content compared to that presented in the fresh sample and, hence, a lower viscosity. The addition of the catalyst enhanced hydrogenation and hydrocracking reaction and therefore resulted in an even lower viscosity, 43%. The addition of water to samples S4−S7 resulted in an increase in the viscosity, which corresponds well with the relatively higher C28−C33 content and other heavier products formed because of free-radical addition reactions in those samples. The liquid viscosity is reported to correlate well with the molecular mass or apparent molecular mass due to aggregation.43 Upon a comparison of S2, S4, and S5, it is clear that the addition of water caused a reduction in the hydrogenation/hydrocracking reactions with acceleration in the reactions that led to the formation of much heavier molecules. The addition of the catalyst in S5 enhanced the hydrogenation reaction but with an end result of almost no overall change in the oil viscosity. A similar trend can be observed by comparing runs S6 and S7. 3.4. Sulfur Reduction. The elemental sulfur results of the samples are represented in Table 5. The results showed a trend Table 5. Results of Sulfur Analysis sample

sulfur amount, wt %

percent desulfurization, %

S1 S2 S3 S4 S5 S6 S7

4.49 3.81 3.05 4.39 4.20 4.13 4.30

NA 15.14 32.07 2.23 6.46 4.23 8.02

similar to that of viscosity reduction. A 32% reduction was achieved with S3, followed by S2 with 15% desulfurization. The hydrogen addition process without water displayed the highest desulfurization compared with the aquathermolysis process without catalyst, 8% for S6, and with catalyst, 4% for S7. Upon comparison of the hydrogen addition runs S2−S4, it is evident that the presence of water inhibits the desulfurization process. Water is known to inhibit desulfurization reactions during upgrading processes.44 The difference in the percent desulfurization of the two runs S2 (15%) and S4 (2%) shows a clear inhibition of the process by water. When a catalyst was used in the presence of water in run S5, the desulfurization was improved from 2% (S4) to 6% (S5). The aquathermolysis runs S6 and S7 displayed low desulfurization with 4.2% for S6 and 8% for S7, indicating an improvement of about 4% when a catalyst was added to run S6, although there is no meaningful differences for the aquathermolysis runs within experimental I


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Figure 12. XRD patterns of the solid precipitate recovered from the reaction of S3.

Figure 13. High-resolution SEM image of the solid precipitate recovered from reaction S3 at different magnifications: (a) 350× (10 μm); (b) 1000× (10 μm); (c) 6000× (1 μm); (d) 37000× (100 nm).

known conventional desulfurization processes used in the refineries for removal of sulfur in crude oil fractions. Sulfur can be removed from thiophenes and mercaptans present in crude oils in the form of hydrogen sulfide, especially when catalysts are used. This is unlikely to take place in aquathermolysis

loss in desulfurization is more likely due to the formation of heavier products by radical addition reactions. In summary, the runs with hydrogen showed higher desulfurization than the aquathermolysis process. The hydrogen addition processes (like hydrodesulfurization) are the J


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oxides, because many metals were also identified from the EDS results. Some of the oxides like silica are inherent to the reservoir. Another element with a high signal is nitrogen, which is also present in the original crude oil because heavy components contain nitrogen. This indicates that the upgrading reactions conducted on the oil samples led to heteroatom removal (oxygen and nitrogen). The DES (ChCl/urea) used is also another source of nitrogen because of the urea present in the reaction medium. Decomposition of the DES during the reaction might lead to the formation of nitrogen-containing compounds. The high signal of chlorine observed in the solid precipitate from chemical analysis can also be traced to ChCl of the DES. Some dissolved salts in crude oils contain chlorine in the form of metal salts of chlorine. Iron and nickel can be explained as originally present in the crude oil. Their presence in recovered solid indicates a reduction of the heavy metal content after the upgrading reactions. The molybdenum and sulfur signals were also observed as expected. The overlapping of the signals for the two observed in both zones (spectra 1 and 5) gives an indication of the interaction of the two as in MoS2. Almost the same amounts of molybdenum were found in the two regions (spectra 1 and 5), which is an indication of the uniform distribution of the catalyst in the system.

because of the lack of hydrogen. Although hydrogen can be produced in situ in aquathermolysis via a water−gas shift reaction, it is not present in large amounts to lead to hydrodesulfurization. 3.5. Solid Precipitate Characterization. SEM−EDS and XRD were carried out on the solid precipitate recovered from reaction sample S3. The yield of the solid product recovered was 7.6 wt %. SEM is useful in exploring the morphology and size of the catalytic particles, while EDS, being a semiquantitative technique, can be used for chemical analysis. XRD can be used for phase identification and crystal structure elucidation. Figure 12 shows the X-ray diffractogram of the solid particles. Despite the noise observed in the spectrum, broader peaks can be clearly seen, which indicates small crystal sizes. Although broader peaks mean small crystal sizes, the XRD pattern indicates the presence of stacking layers of slabs of MoS2 crystallites, which may indicate a high interplanar distance (d spacing). The peaks at 2θ = 33.02°, 40.6097°, 58.5848°, and 58.7203° corresponding to the intensity counts of 149.66, 24.49, 143.26, and 108.43, respectively, are attributed to the presence of MoS2.47,48 The peak at 2θ = 23° indicates the presence of MoO2, which might have been formed from the reduction of MoO3.49 MoS2 is known to be an active form of a molybdenum-based hydrotreating catalyst. Its presence in the produced solid samples after the upgrading process is an indication of its generation in situ. Many researchers have reported that MoS2 can be transformed in situ from relevant molybdenum precursors under conditions similar to those used in this study, in sulfur-containing crude oils.16,48,50−52 The sulfide was generated after decomposition of DES/MoO3 to give dispersed and reactive MoO3, which was subsequently sulfided by the sulfur in the oil. The precise mechanism of this transformation is still unclear, but direct reduction of the salt by hydrogen sulfide seems probable.14,17,51,52 The presence of four different reflections attributable to MoS2 versus one for MoO2 with no characteristic peak of MoO3 is an indication of the good sulfiding behavior of the precursor. The identification of one reflection is not enough to confirm the presence of MoO2 because the reduction of MoO3 to MoO2 requires severe operating conditions and high partial pressures of hydrogen. Figures 13 and S2 and S3 in the SI show the SEM images and results of chemical analysis from EDS. Figure 13a shows the SEM image of the particles (350× magnification), while Figure S3b in the SI was acquired at higher magnification (1000×). When a smaller image (1 μm) was obtained at 6000× magnification, a spherical type particle was observed. Figure 13d shows the image (100 nm) at a magnification of 37000×, in which an estimation of the size of the particle was made. 1.37 μm was found to be the average size of the particles of the agglomerate formed after the reaction. The EDS results, images, and chemical analysis are shown in Figures S2 and S3 in the SI. The SEM image in Figure S2 in the SI shows five different spectra (1−5) in which spectra 1 and 5 were chosen for chemical analysis using the EDS software. The reason is due to the presence of a high concentration of metals in those zones. The chemical analysis and image of spectrum 1 can be seen in Figure S3 in the SI. The tables in the two figures represent the results of chemical analysis. Carbon, chlorine, and oxygen display high signals observed from the spectra. Oxygen and nitrogen are originally present in the heavy crude oil in the form of heteroatoms in constituents like asphaltenes. Oxygen can also be attributed to the presence of oxides, especially metal

4. CONCLUSION Catalytic upgrading of an Omani heavy crude oil was conducted under different conditions simulating hydrocracking and aquathermolysis processes. Some runs of the upgrading reactions were carried out in the presence of catalyst formulated by dissolving MoO3 in DES based on ChCl/urea. Results from the catalyst characterization study show that the incorporation of MoO3 in the DES did not significantly change the structure of the DES, but slight changes in some physicochemical properties were observed. The best of all of the runs is the hydrogenation run with catalyst in which water was not added. It gave the best performance in terms of sulfur, viscosity, and heavy metal reduction. XRD and SEM−EDS analysis of the solid recovered from this run showed the presence of MoS2, an active form of the catalyst, suggestive of its in situ generation. The presence of water in the reaction medium led to an increase rather than a decrease in the viscosity. This negative effect of water in the reaction medium involving this heavy oil, in particular, made the aquathermolysis process (in the presence of water) an undesired route in upgrading this crude oil under the condition studied. Overall, the catalyst is effective in upgrading the crude oil via the hydrogenation route; also the hydrocracking runs performed better than the aquathermolysis runs.

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ASSOCIATED CONTENT

S Supporting Information *

Results of physical property measurements and experimental errors, bar chart comparison of the GC−MS results between the seven reacted oil samples and standards of saturated hydrocarbons, and SEM−EDS results of recovered solids. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +968 2414 2557. Fax: +968 2414 1354. E-mail: [email protected]. K


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Industrial & Engineering Chemistry Research Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank The Research Council of Oman for providing financial support to this work through Research Grant RC/ENG/PCED/11/01. The authors are also grateful to Sultan Qaboos University Oman for allowing use of their facilities and other support offered.

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